Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations

[Peter Zijlstra]

On Thu, Oct 10, 2024 at 12:26:57PM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-10-10 12:03:08 [+0200], Peter Zijlstra wrote:
> > > 
> > > I can't deselect CONFIG_PREEMPT_RCU=y. This is because LAZY selects
> > > PREEMPT_BUILD and PREEMPT_RCU selects itself once PREEMPTION is on.
> > 
> > Oh, the entry isn't user selectable? Fix that perhaps?
> > 
> > -	bool
> > +	bool "Use preemptible RCU"
> > 
> > Or something along those lines -- I forever forget how Kconfig works.
> 
> Oh. Well, yes. If we do this then it becomes suddenly selectable and
> half of the series makes sense…
> But as you said, this complicates things. 

But then you leave it up to the user, instead of doing something quite
random. This would allow you to configure PREEMPT_RCU=n despite also
using PREEMPT_DYNAMIC if that is your thing.

I fundamentally hate the whole randomness of the earlier proposed
selection criteria. It only disables PREEMPT_RCU if you use LAZY and not
also have PREEMPT_RT or PREEMPT_DYNAMIC.